AM8530H AMD [Advanced Micro Devices], AM8530H Datasheet - Page 136

• Manufacturer Part Number: AM8530H
• Description: Serial Communications Controller
• Manufacturer: AMD [Advanced Micro Devices]

AMD
6.2.13
WR12 contains the lower byte of the time constant for the baud rate generator. The time
constant can be changed at any time, but the new value does not take effect until the next
time the time constant is loaded into the down counter. No attempt is made to synchro-
nize the loading of the time constant into WR12 and WR13 with the clock driving the
down counter. For this reason, it is advisable to disable the baud rate generator while the
new time constant is loaded into WR12 and WR13. Ordinarily, this is done anyway to pre-
vent a load of the down counter between the writing of the upper and lower bytes of the
time constant.
The formula for determining the appropriate time constant for a given baud is shown be-
low with the desired rate in bits per second and the BR clock period in seconds. This for-
mula is derived because the counter decrements from N down to ‘0’-plus-one-cycle for
reloading the time constant and is then fed to a toggle flip-flop to make the output a
square wave. Bit positions for WR12 are shown in Figure 6–14.
6.2.14
WR13 contains the upper byte of the time constant for the baud rate generator. Bit posi-
tions for WR13 are shown in Figure 6–15.
6–26
Write Register 12 (Lower Byte of Baud Rate
Generator Time Constant)
Write Register 13 (Upper Byte of Baud Rate
Generator Time Constant)
Time Constant =
D
7
D
6
Figure 6–14. Write Register 12
D
5
D
4
D
2 (Desired Rate) (Baud Rate)
3
D
2
D
1
D
0
TC
TC
TC
TC
TC
TC
TC
TC
1
0
1
2
3
4
5
6
7
Lower Byte of
Time Constant
Register Description
–2